A term applied to rapid contraction or TREMOR of muscles, and especially to a form of abnormal action of the heart muscle in which individual bundles of ﬁbres take up independent action. It is believed to be due to a state of excessive excitability in the muscle associated with the stretching which occurs in dilatation of the heart. The main causes are ATHEROSCLEROSIS, chronic rheumatic heart disease and hypertensive heart disease (see HEART, DISEASES OF). Fibrillation is distinguished as atrial or ventricular, depending on whether the muscle of the atria or of the ventricles is aﬀected. In atrial ﬁbrillation, the heartbeats and the pulse become extremely irregular, both as regards time and force; when the atrium is ﬁbrillating there is no signiﬁcant contraction of the atrial muscle but the cardiac output is maintained by ventricular contraction. In ventricular ﬁbrillation there is no signiﬁcant contractile force, so that there is no cardiac output. The commonest cause is myocardial infarction. Administration of DIGOXIN, timolol or verapamil may restore normal rhythm, and in some patients, CARDIOVERSION – a controlled direct-current electric shock given via a modiﬁed deﬁbrillator placed on the chest wall – is eﬀective.
Article Categories:Medical Dictionary